Dual-Waveband Modelling of Light Dilution for Accurate Quantification of SO2 Emission Rate Measurements from Masaya Volcano, Nicaragua

Abstract:

Ultraviolet measurements of volcanic SO₂ emission rate underpin the quantification of all volcanic gas emissions through their combination with in-plume gas ratios. Challenging volcanic terrain means these measurements can be taken several kilometres away from the plume, leaving them vulnerable to photons scattering between the plume and instrument, commonly called light dilution. Previous work has corrected radiative transfer effects using Markov chain Monte-Carlo methods, but these corrections are not widespread in the volcanological community due to the need for prior knowledge or iteration to obtain plume, atmospheric and measurement conditions.

We combine intensity fitting (iFit) with modelling of simulated spectra using two wavelength windows, specifically 306-316 nm and 312-322 nm, to examine and correct the effect of photon scattering between the plume and instrument, only using information available in the spectra. Our modelling demonstrates low SO₂ column density measurements can be heavily diluted, yet show fitting residuals of a similar magnitude to normal fitting errors. It also shows an upper limit on retrieved column density that decreases with increasing dilution. We then apply our model to correct empirical observations from Masaya volcano, Nicaragua, showing underestimates of SO₂ column density reaching a factor of 5. This causes a large decrease in the observed emission rate of the volcano. If light dilution were not systematically corrected, emission rates of all volcanic volatiles may significantly underestimated, especially at volcanoes that have a large distance between their instrumentation and a plume.